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Electrical conductivity of stretched polypyrrole film
Synthetic Metals, 53 (1993) 245-249
245
Short Communication
Electrical conductivity of stretched polypyrrole film J e o n g H w a n Lee a n d In J a e Chung* Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusung-Dong, Yusung-Gu, Taejon 305-701 (South Korea)
(Received May 7, 1992; in revised form June 15, 1992; accepted July 13, 1992)
Abstract Polypyrrole films with high conductivity and stretchability were synthesized by an electrochemical method in acetonitrile solution at low temperature and low current density. The films were stretched to over twice their original length. The variation of stretching direction conductivity (a~) for a stretched film was evaluated directly by in situ measurements of resistance and strain during the stretching of the polypyrrole film. The calculated values of ¢r~ agreed very well with experimental conductivity data measured by the four-probe technique after drying in air.
Introduction C o n d u c t i n g p o l y m e r s h a v e a t t r a c t e d a g r e a t deal o f r e s e a r c h effort in r e c e n t y e a r s b e c a u s e o f t h e i r intrinsic electrical c o n d u c t i v i t y [ 1 ]. P o l y p y r r o l e is i n t e r e s t i n g as o n e o f t h e c o n d u c t i n g p o l y m e r s b e c a u s e it is stable with high c o n d u c t i v i t y a n d easily s y n t h e s i z e d b y e l e c t r o c h e m i c a l p o l y m e r i z a t i o n . After its i n t r o d u c t i o n [2], g o o d quality films with relatively high c o n d u c t i v i t y w e r e first p r e p a r e d b y K a n a z a w a et al. [3] u s i n g a n e l e c t r o c h e m i c a l technique. H o w e v e r , t h e c o n d u c t i v i t y level ( ~ 100 S c m -1) r e m a i n e d u n c h a n g e d until the e n h a n c e m e n t o f e l e c t r i c a l c o n d u c t i v i t y w a s r e p o r t e d b y O g a s a w a r a et al. [4]. T h e y r e p o r t e d t h a t a highly c o n d u c t i v e a n d s t r e t c h a b l e film could be o b t a i n e d b y a n o d i c o x i d a t i o n o f p y r r o l e in p r o p y l e n e c a r b o n a t e solution at low t e m p e r a t u r e a n d t h a t t h e c o n d u c t i v i t y o f the film w a s e n h a n c e d to 1005 S c m - 1 b y s t r e t c h i n g t o 2.2 t i m e s the original length. The s t r e t c h e d p o l y p y r r o l e films w e r e widely i n v e s t i g a t e d to e x p l a i n the c o n d u c t i v i t y m e c h a n i s m a n d the effect o f c o u n t e r i o n a n d m e c h a n i c a l s t r e t c h on the electrical c o n d u c t i v i t y [ 4 - 8 ] . H o w e v e r , t h e c o n d u c t i v i t y v a r i a t i o n o f the film d u r i n g the stretching has not been studied by in situ measurement. In this study, w e o b s e r v e the v a r i a t i o n o f c o n d u c t i v i t y ((rl) for t h e s t r e t c h e d film c h e c k i n g directly b y i n s i t u m e a s u r e m e n t o f the r e s i s t a n c e *Author to whom correspondence should be addressed.
0379-6779/93/$6.00
© 1993-Elsevier Sequoia. All rights reserved
246 in acetonitrile solvent. T h e p o l y p y r r o l e films are synthesized e l e c t r o c h e m i c a l l y b y passing a c u r r e n t b e t w e e n two platinum e l e c t r o d e s in a solution c o n t a i n i n g 1 vol.% water, acetonitrile solvent, p - t o l u e n e s u l f o n i c acid electrolyte and pyrrole monomer.
Experimental
Polymerization P o l y p y r r o l e was p r e p a r e d b y e l e c t r o c h e m i c a l o x i d a t i o n o n a platinum e l e c t r o d e in a o n e - c o m p a r t m e n t cell which c o n t a i n e d 0.1 M o f p y r r o l e and 0.1 M o f p - t o l u e n e s u l f o n i c acid in acetonitrile c o n t a i n i n g 1 vol.% o f water. T h e films were g r o w n to a c h a r g e density of 10 c o u l o m b c m -2 at - 2 0 °C u n d e r dry n i t r o g e n a t m o s p h e r e and had t h i c k n e s s e s in the r a n g e 4 5 - 4 7 / ~ m . Pyrrole as a m o n o m e r was purified by v a c u u m distillation b e f o r e use. Acetonitrile and p - t o l u e n e s u l f o n i c acid w e r e c o m m e r c i a l l y available and s e c o n d a r y distilled w a t e r was u s e d as an additive solvent. The electrolyte solution had b e e n p u r g e d for 20 rain with dry n i t r o g e n p r i o r to the addition of m o n o m e r s .
Measurements The electrical c o n d u c t i v i t y of the p o l y m e r films was m e a s u r e d b y a fourp r o b e d.c. technique: Within the s t r e t c h i n g a p p a r a t u s the r e s i s t a n c e of the film was m e a s u r e d using a t w o - p r o b e technique. B o t h e n d s o f the film were c o n n e c t e d to c o p p e r wires with an aluminium foil h o l d e r b y p r e s s u r e contact. The ratio of c o n t a c t r e s i s t a n c e to film r e s i s t a n c e was less t h a n 1%.
Stretching A film (45 m m x 2.5 m m x 45 /zm) was s t r e t c h e d with an Instron tensile t e s t e r in acetonitrile solution at 60 °C. T h e load--elongation c u r v e was o b t a i n e d during the s t r e t c h i n g and, at the same time, the r e s i s t a n c e o f the films was c h e c k e d b y using a t w o - p r o b e technique. F o r the p o l y p y r r o l e , n o significant c h a n g e o f c o n d u c t i v i t y b e t w e e n m e a s u r e m e n t s in and out o f the solvent a p p e a r e d . After stretching, the film was s u b s e q u e n t l y s u b j e c t e d to the s t r e t c h e d state until the stress was c o m p l e t e l y relaxed. The c o n d u c t i v i t y was m e a s u r e d after the s t r e t c h e d film was dried in air.
R e s u l t s and d i s c u s s i o n It is a m a t t e r o f general a g r e e m e n t that the m e c h a n i c a l p r o p e r t i e s of c o n d u c t i n g p o l y m e r s are affected b y the p r e p a r a t i o n conditions, such as c u r r e n t density, r e a c t i o n t e m p e r a t u r e , kind of electrolyte, solvent and so on [4, 6, 9]. Hence, the effect o f synthetic c o n d i t i o n s on the elongation ratio was investigated to obtain highly drawn p o l y p y r r o l e film. Figures 1 (a) and (b) s h o w the d e p e n d e n c e of c o n d u c t i v i t y and elongation in acetonitrile solution on the c u r r e n t density and synthetic t e m p e r a t u r e . The film p r e p a r e d in the r a n g e of c u r r e n t density 0 . 2 - 1 . 5 mA c m -2 at
247 120
250
7
80
40
50
' 2
0
' 4
0
' 6
0
i
i
2
4
Current Density [mA-cm "2]
Current Density [mA-cm -2]
(b)
(a)
Fig. 1. D e p e n d e n c e o f (a) c o n d u c t i v i t y a n d (b) e l o n g a t i o n o n c u r r e n t d e n s i t y a n d s y n t h e t i c t e m p e r a t u r e : Q, - 2 0 °C; A , 0 °C; m , 2 0 °C
rr
4 O
n.J
O .J
/
@
n-
0
0
2
i
i
i
25
50
75
0
100
Elongation [%]
Fig. 2. P l o t s o f l o a d - e l o n g a t i o n c u r v e a n d r e s i s t a n c e a s a f u n c t i o n o f e l o n g a t i o n ; d r a w i n g rate o f 0.1 m m rain-1 in acetonitrile at 6 0 °C.
20 °C has a relatively high conductivity of 2 0 0 - 2 3 0 S c m - ' and elongation of 105%. The films pr e pa r ed at higher current density or at high t e m p e r a t u r e are not flexible and show low conductivities ( ~ 100 S c m - ' ) . The reason for these p h e n o m e n a is explained by the side reaction due to over-voltage or thermal motion of molecules [6]. Figure 2 shows the resistance variation of a film and l oad-el ongat i on curve during the drawing of a film in acetonitrile. There are two regions on the l o a d - e l o n g a t i o n curve of the polypyrrole film. Initially, the resistance slowly rises in an approximately linear m a n n e r as the applied elongation increases. In the second region the load rises less steeply with the strain, but the resistance increases slowly and almost linearly in all the region and does not show two regions c o m p a r e d with the l oad-el ongat i on curve. It is recognized that the conductivity ((r,) of polypyrrole increases with elongation. -
248
The load-elongation curve shows typical homogeneous deformation where the load steadily increases with increasing elongation. We now make an assumption of constant volume during the deformation, which is for the polymer deformation. If we define l / l o = l + e and put A l = A o l o , then A = (Aolo)/l. Here, e is the elongation per unit length and A and 1 are crosssectional area and length, respectively. Subscript 0 represents the undrawn film. The conductivity is given by R0
(]-e) 2
(])
where ~1 is the stretching direction conductivity and R is the resistance of the film during elongation. Since the resistance is measured during elongation, we can predict the conductivity crI by using the above equation. In Fig. 3 the dotted line is the fitting of stretching direction conductivities calculated from the resistance data in Fig. 2. The calculated values agree very well with the experimental values (closed triangles) of (rl measured by the four-probe technique after drying in air. Figure 3 shows the conductivities (~1, ~2) of stretched film dried in air after stretching. The conductivity (~i) of the stretched film increases dramatically but the transverse direction conductivity (~2) decreases with strain. This result is similar to that of Yamaura et al. [7]. The difference between these conductivities results from the anisotropy of the molecular structure in the stretched film. The high conductivity (al) is explained by attributing it to the orientation of the polymer chain [5, 6, 10]. This idea is supported 3.0
~ o
2.7
2
m
:~ 2.4
¢~2.1
1.8
a
O.0
I
1.4 1.8 Elongation Ratio (IJLo)
1.5
2.2
z.3
I
I
I
2.6 2.9 3.2 (Temperature) "1" x 10 [K "j']
3.5
Fig. 3. Conductivities a~ ( A ) and (r2 (m) v s . elongation. The dotted line is the fit of the calculated conductivity in the s t r e t c h e d direction u s i n g eqn. (1) f r o m the data in Fig. 2 ( ~ and ~2 are m e a s u r e d by the f o u r - p r o b e t e c h n i q u e after drying). Fig. 4. Plots of electrical conductivity of a film as a function of t e m p e r a t u r e ; tr0 is the conductivity o f the u n s t r e t c h e d film.
249
by X-ray analysis [4]. The drawn films show anisotropic electrical conductivity and the ratio of qx/q2 for 100% elongated film is about 4. The value of (rl/~2 is less than that of polyacetylene film [10, 11]; polypyrrole has less orientation than polyacetylenes because of the structural hindrance of the pyrrole ring in the polymer. Figure 4 shows plots of log q versus T - 1/4 of the stretched and unstretched films obtained from the four-probe conductivity measurement. The film was stretched up to twice the original length. Linear fitting throughout the temperature range from 80 to 300 K indicates the applicability of Mott's variable range hopping model [12l to the polypyrrole film. This behaviour has also been widely observed in unstretched polypyrrole films [8, 13, 14] and stretched film [8]. The ratio of ax/q2 is independent of temperature range, suggesting that the same hopping process controls conductivities in both directions [8].
Conclusions
The variation of conductivity (ql) for the stretched films was checked directly by in situ measurement of the resistance during the stretching of a polypyrrole film. The ratio of qx/q2 was independent of temperature in the range 8 0 - 3 0 0 K. The temperature dependence of the conductivities, whether stretched or not, followed very well Mott's variable range hopping model.
References 1 T.A. Skotheim (ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986. 2 A. Dall'Olio, Y. Dascola, V. Varacca and V. Bocchi, C. R. Acad. Sci. Paris, Set. C, 257 (1968) 433. 3 K. K. Kanazawa, A. F. Diaz, R. H. Geiss, W. D. Gill, J. F. Kwak, J. A. Logan, J. F. Rabolt and G. B. Street, J. Chem. Soc., Chem. Commun., (1979) 854. 4 M. Ogasawara, K. Funahashi and K. Iwata, Mol. Cryst. Liq. Cryst., 118 (1985) 159. 5 M. Ogasawara, K. Funahashi, T. Demura, T. Hagiwara and K. lwata, Synth. Met., 14 (1986) 61. 6 M. Yamaura, T. Hagiwara and K. Iwata, Synth. Met., 26 (1988) 209. 7 M. Yamaura, T. Hagiwara, M. Hirasaka, T. Demura and K. Iwata, Synth. Met., 28 (1989) C157. 8 K. Sato, M. Yamaura, T. Hagiwara, K. Murata and M. Tokumoto, Synth. Met., 40 (1991) 35. 9 A. F. Diaz and B. Hall, IBM J. Res. Dev., 27 (1983) 342. 10 Y. W. Park, M. A. Druy, C. K. Chiang, A. G. MacDiarmid, A. J. Heeger, H. Shirakawa and S. Ikeda, J. Polym. Sci., Polym. Lett. Ed., 17 (1979) 195. 11 Y. W. Park, C. O. Yoon, C. H. Lee, H. Shirakawa, Y. Suezaki and K. Akagi, Synth. Met., 41-43 (1991) 125. 12 N. F. Mott and E. A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon Press, Oxford, 1979, p. 32. 13 B. Lundberg and B. Sundqvist, Mol. Cryst. Liq. Cryst., 118 (1985) 155. 14 D. S. Maddison, J. Unsworth and R. B. Roberts, Synth. Met., 41-43 (1991) 401.
245
Short Communication
Electrical conductivity of stretched polypyrrole film J e o n g H w a n Lee a n d In J a e Chung* Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusung-Dong, Yusung-Gu, Taejon 305-701 (South Korea)
(Received May 7, 1992; in revised form June 15, 1992; accepted July 13, 1992)
Abstract Polypyrrole films with high conductivity and stretchability were synthesized by an electrochemical method in acetonitrile solution at low temperature and low current density. The films were stretched to over twice their original length. The variation of stretching direction conductivity (a~) for a stretched film was evaluated directly by in situ measurements of resistance and strain during the stretching of the polypyrrole film. The calculated values of ¢r~ agreed very well with experimental conductivity data measured by the four-probe technique after drying in air.
Introduction C o n d u c t i n g p o l y m e r s h a v e a t t r a c t e d a g r e a t deal o f r e s e a r c h effort in r e c e n t y e a r s b e c a u s e o f t h e i r intrinsic electrical c o n d u c t i v i t y [ 1 ]. P o l y p y r r o l e is i n t e r e s t i n g as o n e o f t h e c o n d u c t i n g p o l y m e r s b e c a u s e it is stable with high c o n d u c t i v i t y a n d easily s y n t h e s i z e d b y e l e c t r o c h e m i c a l p o l y m e r i z a t i o n . After its i n t r o d u c t i o n [2], g o o d quality films with relatively high c o n d u c t i v i t y w e r e first p r e p a r e d b y K a n a z a w a et al. [3] u s i n g a n e l e c t r o c h e m i c a l technique. H o w e v e r , t h e c o n d u c t i v i t y level ( ~ 100 S c m -1) r e m a i n e d u n c h a n g e d until the e n h a n c e m e n t o f e l e c t r i c a l c o n d u c t i v i t y w a s r e p o r t e d b y O g a s a w a r a et al. [4]. T h e y r e p o r t e d t h a t a highly c o n d u c t i v e a n d s t r e t c h a b l e film could be o b t a i n e d b y a n o d i c o x i d a t i o n o f p y r r o l e in p r o p y l e n e c a r b o n a t e solution at low t e m p e r a t u r e a n d t h a t t h e c o n d u c t i v i t y o f the film w a s e n h a n c e d to 1005 S c m - 1 b y s t r e t c h i n g t o 2.2 t i m e s the original length. The s t r e t c h e d p o l y p y r r o l e films w e r e widely i n v e s t i g a t e d to e x p l a i n the c o n d u c t i v i t y m e c h a n i s m a n d the effect o f c o u n t e r i o n a n d m e c h a n i c a l s t r e t c h on the electrical c o n d u c t i v i t y [ 4 - 8 ] . H o w e v e r , t h e c o n d u c t i v i t y v a r i a t i o n o f the film d u r i n g the stretching has not been studied by in situ measurement. In this study, w e o b s e r v e the v a r i a t i o n o f c o n d u c t i v i t y ((rl) for t h e s t r e t c h e d film c h e c k i n g directly b y i n s i t u m e a s u r e m e n t o f the r e s i s t a n c e *Author to whom correspondence should be addressed.
0379-6779/93/$6.00
© 1993-Elsevier Sequoia. All rights reserved
246 in acetonitrile solvent. T h e p o l y p y r r o l e films are synthesized e l e c t r o c h e m i c a l l y b y passing a c u r r e n t b e t w e e n two platinum e l e c t r o d e s in a solution c o n t a i n i n g 1 vol.% water, acetonitrile solvent, p - t o l u e n e s u l f o n i c acid electrolyte and pyrrole monomer.
Experimental
Polymerization P o l y p y r r o l e was p r e p a r e d b y e l e c t r o c h e m i c a l o x i d a t i o n o n a platinum e l e c t r o d e in a o n e - c o m p a r t m e n t cell which c o n t a i n e d 0.1 M o f p y r r o l e and 0.1 M o f p - t o l u e n e s u l f o n i c acid in acetonitrile c o n t a i n i n g 1 vol.% o f water. T h e films were g r o w n to a c h a r g e density of 10 c o u l o m b c m -2 at - 2 0 °C u n d e r dry n i t r o g e n a t m o s p h e r e and had t h i c k n e s s e s in the r a n g e 4 5 - 4 7 / ~ m . Pyrrole as a m o n o m e r was purified by v a c u u m distillation b e f o r e use. Acetonitrile and p - t o l u e n e s u l f o n i c acid w e r e c o m m e r c i a l l y available and s e c o n d a r y distilled w a t e r was u s e d as an additive solvent. The electrolyte solution had b e e n p u r g e d for 20 rain with dry n i t r o g e n p r i o r to the addition of m o n o m e r s .
Measurements The electrical c o n d u c t i v i t y of the p o l y m e r films was m e a s u r e d b y a fourp r o b e d.c. technique: Within the s t r e t c h i n g a p p a r a t u s the r e s i s t a n c e of the film was m e a s u r e d using a t w o - p r o b e technique. B o t h e n d s o f the film were c o n n e c t e d to c o p p e r wires with an aluminium foil h o l d e r b y p r e s s u r e contact. The ratio of c o n t a c t r e s i s t a n c e to film r e s i s t a n c e was less t h a n 1%.
Stretching A film (45 m m x 2.5 m m x 45 /zm) was s t r e t c h e d with an Instron tensile t e s t e r in acetonitrile solution at 60 °C. T h e load--elongation c u r v e was o b t a i n e d during the s t r e t c h i n g and, at the same time, the r e s i s t a n c e o f the films was c h e c k e d b y using a t w o - p r o b e technique. F o r the p o l y p y r r o l e , n o significant c h a n g e o f c o n d u c t i v i t y b e t w e e n m e a s u r e m e n t s in and out o f the solvent a p p e a r e d . After stretching, the film was s u b s e q u e n t l y s u b j e c t e d to the s t r e t c h e d state until the stress was c o m p l e t e l y relaxed. The c o n d u c t i v i t y was m e a s u r e d after the s t r e t c h e d film was dried in air.
R e s u l t s and d i s c u s s i o n It is a m a t t e r o f general a g r e e m e n t that the m e c h a n i c a l p r o p e r t i e s of c o n d u c t i n g p o l y m e r s are affected b y the p r e p a r a t i o n conditions, such as c u r r e n t density, r e a c t i o n t e m p e r a t u r e , kind of electrolyte, solvent and so on [4, 6, 9]. Hence, the effect o f synthetic c o n d i t i o n s on the elongation ratio was investigated to obtain highly drawn p o l y p y r r o l e film. Figures 1 (a) and (b) s h o w the d e p e n d e n c e of c o n d u c t i v i t y and elongation in acetonitrile solution on the c u r r e n t density and synthetic t e m p e r a t u r e . The film p r e p a r e d in the r a n g e of c u r r e n t density 0 . 2 - 1 . 5 mA c m -2 at
247 120
250
7
80
40
50
' 2
0
' 4
0
' 6
0
i
i
2
4
Current Density [mA-cm "2]
Current Density [mA-cm -2]
(b)
(a)
Fig. 1. D e p e n d e n c e o f (a) c o n d u c t i v i t y a n d (b) e l o n g a t i o n o n c u r r e n t d e n s i t y a n d s y n t h e t i c t e m p e r a t u r e : Q, - 2 0 °C; A , 0 °C; m , 2 0 °C
rr
4 O
n.J
O .J
/
@
n-
0
0
2
i
i
i
25
50
75
0
100
Elongation [%]
Fig. 2. P l o t s o f l o a d - e l o n g a t i o n c u r v e a n d r e s i s t a n c e a s a f u n c t i o n o f e l o n g a t i o n ; d r a w i n g rate o f 0.1 m m rain-1 in acetonitrile at 6 0 °C.
20 °C has a relatively high conductivity of 2 0 0 - 2 3 0 S c m - ' and elongation of 105%. The films pr e pa r ed at higher current density or at high t e m p e r a t u r e are not flexible and show low conductivities ( ~ 100 S c m - ' ) . The reason for these p h e n o m e n a is explained by the side reaction due to over-voltage or thermal motion of molecules [6]. Figure 2 shows the resistance variation of a film and l oad-el ongat i on curve during the drawing of a film in acetonitrile. There are two regions on the l o a d - e l o n g a t i o n curve of the polypyrrole film. Initially, the resistance slowly rises in an approximately linear m a n n e r as the applied elongation increases. In the second region the load rises less steeply with the strain, but the resistance increases slowly and almost linearly in all the region and does not show two regions c o m p a r e d with the l oad-el ongat i on curve. It is recognized that the conductivity ((r,) of polypyrrole increases with elongation. -
248
The load-elongation curve shows typical homogeneous deformation where the load steadily increases with increasing elongation. We now make an assumption of constant volume during the deformation, which is for the polymer deformation. If we define l / l o = l + e and put A l = A o l o , then A = (Aolo)/l. Here, e is the elongation per unit length and A and 1 are crosssectional area and length, respectively. Subscript 0 represents the undrawn film. The conductivity is given by R0
(]-e) 2
(])
where ~1 is the stretching direction conductivity and R is the resistance of the film during elongation. Since the resistance is measured during elongation, we can predict the conductivity crI by using the above equation. In Fig. 3 the dotted line is the fitting of stretching direction conductivities calculated from the resistance data in Fig. 2. The calculated values agree very well with the experimental values (closed triangles) of (rl measured by the four-probe technique after drying in air. Figure 3 shows the conductivities (~1, ~2) of stretched film dried in air after stretching. The conductivity (~i) of the stretched film increases dramatically but the transverse direction conductivity (~2) decreases with strain. This result is similar to that of Yamaura et al. [7]. The difference between these conductivities results from the anisotropy of the molecular structure in the stretched film. The high conductivity (al) is explained by attributing it to the orientation of the polymer chain [5, 6, 10]. This idea is supported 3.0
~ o
2.7
2
m
:~ 2.4
¢~2.1
1.8
a
O.0
I
1.4 1.8 Elongation Ratio (IJLo)
1.5
2.2
z.3
I
I
I
2.6 2.9 3.2 (Temperature) "1" x 10 [K "j']
3.5
Fig. 3. Conductivities a~ ( A ) and (r2 (m) v s . elongation. The dotted line is the fit of the calculated conductivity in the s t r e t c h e d direction u s i n g eqn. (1) f r o m the data in Fig. 2 ( ~ and ~2 are m e a s u r e d by the f o u r - p r o b e t e c h n i q u e after drying). Fig. 4. Plots of electrical conductivity of a film as a function of t e m p e r a t u r e ; tr0 is the conductivity o f the u n s t r e t c h e d film.
249
by X-ray analysis [4]. The drawn films show anisotropic electrical conductivity and the ratio of qx/q2 for 100% elongated film is about 4. The value of (rl/~2 is less than that of polyacetylene film [10, 11]; polypyrrole has less orientation than polyacetylenes because of the structural hindrance of the pyrrole ring in the polymer. Figure 4 shows plots of log q versus T - 1/4 of the stretched and unstretched films obtained from the four-probe conductivity measurement. The film was stretched up to twice the original length. Linear fitting throughout the temperature range from 80 to 300 K indicates the applicability of Mott's variable range hopping model [12l to the polypyrrole film. This behaviour has also been widely observed in unstretched polypyrrole films [8, 13, 14] and stretched film [8]. The ratio of ax/q2 is independent of temperature range, suggesting that the same hopping process controls conductivities in both directions [8].
Conclusions
The variation of conductivity (ql) for the stretched films was checked directly by in situ measurement of the resistance during the stretching of a polypyrrole film. The ratio of qx/q2 was independent of temperature in the range 8 0 - 3 0 0 K. The temperature dependence of the conductivities, whether stretched or not, followed very well Mott's variable range hopping model.
References 1 T.A. Skotheim (ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986. 2 A. Dall'Olio, Y. Dascola, V. Varacca and V. Bocchi, C. R. Acad. Sci. Paris, Set. C, 257 (1968) 433. 3 K. K. Kanazawa, A. F. Diaz, R. H. Geiss, W. D. Gill, J. F. Kwak, J. A. Logan, J. F. Rabolt and G. B. Street, J. Chem. Soc., Chem. Commun., (1979) 854. 4 M. Ogasawara, K. Funahashi and K. Iwata, Mol. Cryst. Liq. Cryst., 118 (1985) 159. 5 M. Ogasawara, K. Funahashi, T. Demura, T. Hagiwara and K. lwata, Synth. Met., 14 (1986) 61. 6 M. Yamaura, T. Hagiwara and K. Iwata, Synth. Met., 26 (1988) 209. 7 M. Yamaura, T. Hagiwara, M. Hirasaka, T. Demura and K. Iwata, Synth. Met., 28 (1989) C157. 8 K. Sato, M. Yamaura, T. Hagiwara, K. Murata and M. Tokumoto, Synth. Met., 40 (1991) 35. 9 A. F. Diaz and B. Hall, IBM J. Res. Dev., 27 (1983) 342. 10 Y. W. Park, M. A. Druy, C. K. Chiang, A. G. MacDiarmid, A. J. Heeger, H. Shirakawa and S. Ikeda, J. Polym. Sci., Polym. Lett. Ed., 17 (1979) 195. 11 Y. W. Park, C. O. Yoon, C. H. Lee, H. Shirakawa, Y. Suezaki and K. Akagi, Synth. Met., 41-43 (1991) 125. 12 N. F. Mott and E. A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon Press, Oxford, 1979, p. 32. 13 B. Lundberg and B. Sundqvist, Mol. Cryst. Liq. Cryst., 118 (1985) 155. 14 D. S. Maddison, J. Unsworth and R. B. Roberts, Synth. Met., 41-43 (1991) 401.